1. Field of the Invention
The present invention relates to an xcex1-alumina powder and a heat-conductive sheet containing the same and, more specifically, to an xcex1-alumina powder suitable for use as a filler manufacturing a heat-conductive sheet.
2. Description of the Related Art
A semiconductor chip generates heat when it operates. Such heat must be radiated from the semiconductor chip to stabilize the operation of the chip. Usually, such a semiconductor chip is encapsulated with an epoxy resin or the like and used in the form of a package. To date, studies have been made to improve the heat conductivity of such an epoxy resin used as a packaging material in pursuit of effective radiation of heat generated from such a semiconductor chip. For instance, filling the epoxy resin with a heat-conductive inorganic powder been proposed as a method of improving the heat conductivity of the epoxy resin.
As the processing speed of semiconductor chips increases with improvements in chip design and manufacturing techniques, the amount of heat generated from such a semiconductor chip increases. Semiconductor chip packages, generally, are provided with a radiating plate composed of a metal to allow heat from the package to efficiently radiate to the outside. Further, such semiconductor packages are sometimes provided with a silicone rubber sheet located between the package and the radiating plate in order to provide a tight contact between the package and the radiating plate. In an attempt to improve the heat conductivity of such a sheet, including an xcex1-alumina powder filler into the silicone rubber has been studied recently.
xcex1-alumina powder, which is produced in a large quantity by, for example, a process of calcining aluminum hydroxide obtained by hydrolysis of an alkali aluminate, is inexpensive and easily available. For this reason, such an aluminum powder is widely used as a material for an abrasive or as a sintering material.
xcex1-alumina powder having a narrow particle size distribution is known from, for example, Japanese Patent Laid-Open No. HEI 6-191833. Using conventional xcex1-alumina powders, as they are, as fillers for silicone rubber sheets, however, is not easy because of difficulty in filling a large amount of such an xcex1-alumina powder into the silicone rubber.
Accordingly, it is an object of the present invention to provide an xcex1-alumina powder suitable for use as a filler for resin or a rubber (hereinafter referred to as xe2x80x9cresin or the likexe2x80x9d) and a heat-conductive sheet containing the same.
The inventors of the present invention have studied xcex1-alumina powders in order to improve their ability to be used as a filler for resin or the like and, as a result, discovered that a novel xcex1-alumina powder having a crystalline form as described herein has a good filling property with respect to the resin or the like, and completed the present invention.
According to one aspect of the present invention, there is provided an xcex1-alumina powder including xcex1-alumina particles having an average diameter of not less than about 2 xcexcm and not more than about 5 xcexcm, a particle size distribution such that a ratio of 90 wt % diameter D90 to 10 wt % diameter D10 represented by D90/D10 is not more than about 2, and a crystalline form represented by an index A of more than about 0.40 and not more than about 0.50, wherein the index A is obtained by substituting X-ray intensities I(110), I(300) and I(116) at the (110) plane, the (300) plane and the (116) plane, respectively, determined by X-ray diffraction, into the following formula (I):
A={I(110)+I(300)}/{2xc3x97I(116)}xe2x80x83xe2x80x83(I).
According to another aspect of the present invention, there is provided a heat-conductive sheet comprising a formed product made from a composition prepared by filling an xcex1-alumina powder described above into resin or a rubber.
The foregoing and other objects, features and attendant advantages of the present invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.